1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly to a high power semiconductor laser device having a small far-field vertical angle (FFV) and excellent optical power efficiency.
2. Description of the Related Art
Generally, semiconductor laser devices are used as light sources in the information-processing and/or optical communication fields, and may be exemplified by an optical pick-up apparatus of an optical disk system such as a CD or DVD. Particularly, the information-processing field requires a technique for decreasing the beam size and increasing the optical density in order to increase the storage density. For example, a conventional semiconductor laser device for CDs has a FFV value of 35°, while a recent semiconductor laser device for CD-RWs should have a FFV value of approximately 14° to approximately 17°.
In order to manufacture such a semiconductor laser device, Japanese Patent Laid-open No. Hei 11-233883 discloses a semiconductor laser device having a nonsymmetrical refractive index profile.
This semiconductor laser device has an improved FFV value and assures higher power by decreasing the optical distribution at a ridged structure and shifting the optical distribution from the ridged structure toward a substrate via the vertically nonsymmetrical refractive profile centering on a nonsymmetrical active layer.
FIG. 1a is a cross-sectional view of this conventional semiconductor laser device. With reference to FIG. 1a, the semiconductor laser device comprises an n-type AlGaAs clad layer 9, a first guide layer 8, an AlGaAs active layer 7, a second guide layer 6 and a p-type AlGaAs clad layer 5, which are sequentially stacked on a GaAs substrate 10. The first and second guide layers 6 and 8 contact the upper and lower surfaces of the active layer 7, and are i(intrinsic)-AlGaAs guide layers with Al content higher than those of the active layer 7.
Here, the p-type AlGaAs clad layer 5 has a ridged structure so that current distribution is concentrated thereon, and a current block layer 4 for cutting off the distribution of the current is formed around the ridged structure.
A p-type GaAs cap layer 3 is formed on the upper surface of the ridged structure. A p-type GaAs layer 2 with a proper thickness is formed on the p-type GaAs cap layer 3 so that the effect of the p-type AlGaAs clad layer 5 on the active layer 7 occurring at a subsequent step is prevented by the p-type GaAs layer 2. A p-type electrode 1 is formed on the p-type GaAs layer 2, and an n-type electrode 11 is formed on the lower surface of the GaAs substrate 10.
Alternatively, an insulating layer instead of the p-type GaAs layer 2 may be formed on the current block layer 4 so that the p-type GaAs cap layer 3 is exposed to the outside, and the p-type electrode 1 may be formed on the insulating layer so that the p-type electrode 1 is electrically connected to the exposed portion of the p-type GaAs cap layer 3.
FIG. 1b is a graph illustrating a refractive index profile relative to the stacking direction of the semiconductor laser device shown in FIG. 1a, i.e., a longitudinal direction. As shown in FIG. 1b, the n-type AlGaAs clad layer 9 has refractivity higher than that of the p-type AlGaAs clad layer 5. That is, differently from the conventional semiconductor laser device with a symmetrical refractive index profile centering on an active layer, this semiconductor laser device has a nonsymmetrical refractive index profile. As shown in FIG. 2, the optical intensity distribution of the semiconductor laser device is improved by the above nonsymmetrical refractive index profile.
With reference to FIG. 2, the optical distribution of the semiconductor laser device is designed so that the light generated from the active layer 7, as indicated by arrows, is shifted from the n-type AlGaAs clad layer 9 toward the GaAs substrate 10. Accordingly, the optical distribution is decreased in the p-type AlGaAs clad layer 5 with relatively low refractivity, but increased in the n-type AlGaAs clad layer 9 on the substrate 10.
As a result, the FFV of the semiconductor laser device becomes narrow, and the aspect ratio (an angle in a longitudinal direction (x)/an angle in a transversal direction (z)) of the beam of the semiconductor laser device is reduced.
However, as shown in FIG. 2, since the center of the optical distribution, i.e., the position (C1) of an optical peak, is shifted toward the GaAs substrate 10, the center of the optical distribution deviates from a central portion (Ca) of the active layer 7 and shifted to the n-type clad layer 9 with high refractivity. In case that the position (C1) of the optical peak deviates from the central portion (Ca) of the active layer 7 generating light by recoupling electrons and holes, the gain efficiency is reduced, thus causing the decrease of optical power.
In order to solve this problem generated by the nonsymmetrical structure of the conventional semiconductor laser device, the refractivity of the second guide layer 6 disposed on the opposite side of the n-type clad layer 9 with high refractivity is heightened, or the thickness or band gap of the second guide layer 6 is increased more than that of the first guide layer 8.
However, such methods for adjusting the second guide layer may change the entire distribution of the laser device beam, or upset the balance of the ratio of holes and electrons injected into the active layer 7 via the p-type clad layer 5 with the ridged structure and the n-type clad layer 9, thus reducing the recoulping efficiency.
Accordingly, there is required a novel structure of a semiconductor laser device with an nonsymmetrical structure in order to reduce a FFV value, in which the deviation of the peak of optical intensity distribution from the central portion of an active layer due to an nonsymmetrical refractive index profile is prevented.